1. Field of the Invention
The present invention relates to a composition and process for use in the manufacture of friction materials. More particularly, this invention relates to a composition capable of being rapidly cured by electromagnetic energy suitable for use in the manufacture of friction materials.
2. Description of the Prior Art
Friction materials for use as brake pads are conventionally made by mixing friction modifiers, reinforcing fibers, and matrixs until a desired blend is achieved. Then, the blend is placed in a mold, compressed to a desired density, and heated in a conventional electric or steam chamber for a fixed time period to complete a reaction between a curing agent and a matrix.
Also known are methods for manufacturing friction materials which do not require the use of an oven. However, such methods have a slow curing rate. See e.g., U.S. Pat. No. 4,617,165 (12 to 24 hour cure). It is also known that the cure time can be reduced to about four hours by evaporating the volatiles present in the pre-cured friction material blend in conventional ovens set at temperatures ranging between about 200.degree. to 400.degree. C. See U.S. Pat. Nos. 4,537,823 and 4,605,595. Although these methods are less time-consuming, the matrix for such materials produced therefrom does not cure uniformly.
Many friction materials produced according to known methods also must undergo a separate scorching process by means of flame and plasma in order to further improve their frictional surface. Not only are such conventional scorching processes expensive, but also the scorched layer resulting therefrom is very thin.
It is known that microwaves induce heating in certain types of compounds. See Newnham, et al., "Fundamental Interaction Mechanisms Between Microwaves and Matter", 21 Ceramic Transactions, Microwave: Theory And Application in Materials Processing 51-68 (1991). The concept of using electromagnetic energy to improve the microstructure of materials, i.e. to produce material which are free from defects such as cracks and voids, has been applied to processes for making formed coke in compacted carbonaceous masses as taught in U.S. Pat. No. 4,412,841. In the area of friction materials, high frequency wave heaters have been used to pre-heat resin sheets at low temperatures in order to decrease their porosity and thus achieve better compaction before they are cut and cured, see U.S. Pat. No. 3,950,149 (grinding wheel production), as well as to heat metal backing plates in order to soften the adhesive layer between the plate and the brake lining attached thereto. See U.S. Pat. No. 4,668,850.
It would be desirable to have a rapidly cureable composition suitable for use in the production of friction materials so that the time for their manufacture can be reduced. It would also be desirable to have such a composition whereby the friction materials comprised thereof would possess properties such as thick, uniformly cured preburnished or scorched sections, selective porosity and adjustable compressibility without sacrificing any of the mechanical properties associated with known friction materials.